1. Field of the Invention
The present invention generally relates to a projection optical system and an image display apparatus.
2. Description of the Related Art
In projection display apparatuses using liquid crystal panels, three liquid crystal panels for red, green, and blue images are illuminated with linearly polarized lights of three wavelength regions of red, green, and blue, respectively. Polarization directions of the lights of three wavelength regions are individually controlled by the liquid crystal panels. An analyzer passes linearly polarized lights having a particular polarization direction and blocks linearly polarized lights having a polarization direction perpendicular to the particular polarization direction. Next, light beams passing through the analyzer are mixed in a color mixing (combining) system and generate an image light. The generated image light is projected onto a screen by a projection optical system.
When entering a color mixing system (e.g., a cross dichroic prism), a light of one wavelength range is an S-polarized light, and a light of another wavelength range is a P-polarized right. That is to say, when the mixed image light enters the projection optical system, linearly polarized light beams of three wavelength regions have different polarization directions.
If light beams of three wavelength regions having different polarization directions enter a projection optical system including reflective optical elements, the white balance on the screen is off, because reflectance of the reflective optical elements varies according to the polarization direction, and therefore transmittance of the projection optical system (reflectance of the reflective optical elements) varies according to the wavelength range.
In order to avoid this, a quarter-wave plate is disposed at the entrance of a projection optical system to reduce the difference in transmittance of the projection optical system between the wavelength ranges.
Japanese Patent Laid-Open No. 2003-121811 discloses a liquid crystal projector in which a quarter-wave plate is disposed between the entrance surface of an image projection lens and the exit surface of a cross dichroic prism for mixing images of three liquid crystal panels in order to convert three-color linearly polarized components into circularly polarized lights or elliptically polarized lights.
The related art disclosed in Japanese Patent Laid-Open No. 2000-235168 eliminates the difference in transmittance and reflectance between image lights output from a liquid crystal projector and having different polarized components, and the difference in transmittance between linearly polarized lights caused by the difference in angle of incidence, and provides a uniformly colored screen. A phase delay plate (quarter-wave plate) is disposed between a color mixing prism and a screen. The plate changes the polarization direction of image lights entering a projection optical system.
In conventional liquid crystal projectors, color lights have different polarization directions in order to enhance the effect of a dichroic prism. Therefore, a polarizing screen can reflect only the light having one of two polarization directions. Therefore, if such liquid crystal projectors use a polarizing screen, one or two color lights are not reflected, and therefore, for example, a green image is displayed, or a magenta (red plus blue) image is displayed. To solve this problem, Japanese Patent Laid-Open No. 2000-098322 discloses a liquid crystal projector including three LCD elements, a dichroic prism, and a projection lens. The LCD elements output linearly polarized lights. The prism mixes three color image lights output from the LCD elements. The projection lens magnifies and projects the mixed image lights. One of the three color lights has a polarization direction perpendicular to those of the other color lights. A quarter-wave plate is disposed on the exit side of the prism.
U.S. Pat. No. 6,183,091, and Japanese Patent Laid-Open Nos. 2003-057600, 2003-066373, and 2003-075769 disclose related arts in which a wavelength-selective half-wave plate is provided on the entrance side of a projection optical system to make all of the image lights entering the projection optical system have the same polarization direction.
In Japanese Patent Laid-Open Nos. 2003-121811, 2000-235168, and 2000-098322, light beams entering a projection optical system are circularly polarized lights. Therefore, when a reference axis ray is defined as a ray passing through the center of a display element, the center of a diaphragm, and the center of a screen, the reference axis ray includes an S-polarized component and a P-polarized component with respect to the plane of incidence on an optical reflective surface. In a reflective surface, reflectance of a P-polarized light is lower than that of an S-polarized light. Therefore, when a circularly polarized light enters a projection optical system including a reflective surface, loss of light quantity occurs. This is the same with lights other than the reference axis ray. Since they include a P-polarized component, reflectance deteriorates. Therefore, loss of the total quantity of light increases.
In U.S. Pat. No. 6,183,091, and Japanese Patent Laid-Open Nos. 2003-057600, 2003-066373, and 2003-075769, all of the image lights entering a projection optical system have the same polarization direction. However, a projection optical system including reflective surfaces is not discussed in these documents. If a projection optical system includes reflective surfaces and P-polarized lights are incident on the reflective surfaces, reflectance deteriorates.